I prefer DUET due to its slightly more laid back upper midrange and much less background noise. 720 is an awesome amp/DAC but if you use earphones, you will hear the hiss, no matter the make. The DUET is nearly silent and also has great control over the usable volume range through its volume pot. The 720 is more aggressive. If I was using headphones only, I'd probably prefer the DT880 with the DUET and the HD600 with the 720. But that is preference. Both are very high performance amps, but the DUET has a much quieter background. I can't pass that up. If you primarily use headphones either one will be great. Of course, if you don't need the DAC, or if you need it, there is only one choice for you.
Me? I'm a DUET guy as most of my battery headphone amp listening is done portably, and with earphones. Lieven is probably more of a home listener. And my ears have been called 'hiss kings', meaning, I hear hiss with grave alacrity. It's a curse.